Lactic acid is common to all the solids and fluids of the system. It is found united with potash, soda, lime, or magnesia.
D.
The word duodenum is derived from the Latin, signifying “twelve,” since the intestine, of which this is the name, is usually about twelve fingers’ breadth in length. The jejunum is also from the Latin jejunum, empty, since it is usually found in that condition after death, as the food seems to pass rapidly through this part of the intestine. The term ileum is from the Greek, signifying “to twist,” since it always appears in a contorted condition. The name cæcum is derived from the fact of its being a blind or short sack, perforated by the extremity of the ileum. The name of the next division of the intestine—colon—is from the Greek, “to prohibit,” as the contents of the alimentary canal pass slowly through this portion. The rectum is named from the straight direction that it assumes in the latter part of its course.
E.
The food is forced through the alimentary canal by contractions of its muscular coat, produced by the nervous filaments of the sympathetic system, not being at all dependent on the cerebro-spinal centre. This is called the peristaltic, or vermicular motion. The great length of intestine in all animals, and especially in the herbivorous ones, is owing to the necessity of exposing the food to a large number of the lacteals, that the nourishment may all be taken from it.
F.
The different processes through which the food passes before assimilation are of considerable interest. The mastication and mixture of the saliva with the food are purely of a mechanical nature. When any solid or fluid substance is placed upon the tongue, or in contact with the inner surface of the cheeks, by an involuntary act, the salivary glands are stimulated to activity, and commence pouring the saliva into the mouth through the salivary ducts. As soon as mastication commences, the contraction of the masseter and other muscles employed in mastication stimulates the salivary glands to increased action, and a still greater quantity of saliva is secreted and forced upon the food, which is constantly being ground to a finer condition, until it is sufficiently reduced for deglutition.
Whether the salivary fluid acts any other part than simply that of a demulcent to assist the gastric juice in still further dissolving the food, is yet a matter of some doubt, although it is found that no other liquid will equally well subserve the process of digestion and promote health.
After the food is in the condition ready to be swallowed, by an apparently involuntary motion, it is placed upon the back of the tongue, which carries it backwards to the top of the pharynx, where the constrictions of the pharynx, aided by the muscles of the tongue and floor of the mouth, with a sudden and violent movement thrust it beyond the epiglottis, in order to allow the least necessary time to the closure of the glottis, after which, by the compression of the œsophagus, it is forced into the stomach.